1. Field of the Invention
The present invention relates generally to the field of telecommunications, and more particularly to the field of wireless communications.
2. Discussion of the Background Art
Wireless communication systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Since 1991, the availability of TDMA (Time Division Multiple Access) digital cellular and Personal Communication Systems (PCS) technologies has led to wide adoption of wireless communications. In 1995, Code Division Multiple Access (CDMA) technology was introduced and is now used in PCS and cellular systems as well. Most recently, great interest and effort has been focussed on deploying a Wideband CDMA (W-CDMA) system in the wireless communication market. The most recent effort at standardizing W-CDMA resulted in the IMT-2000 standard, to be deployed by year 2002 to 2004, as discussed in Tero Ojanpera and Ramjee Prasad, Wideband CDMA for Third Generation Mobile Communications, Artech House, 1998, and Jong Sam Lee and Leonard E. Miller, CDMA Systems Engineering Handbook, Artech House, 1998, which are incorporated herein by reference.
In general, FIG. 1 illustrates a transceiver in a wireless communications system. A desired signal is subject to multipath fading (slow or fast) introduced by communication channel medium 72, which manifests itself as interference among many signal paths, each path having a different time delay. To overcome the communication channel interference and its effect on the signal constellation, pilot symbols are transmitted within the data symbols so as to estimate the fading channel characteristics and reduce signal errors. Fading may include long-term fading due to variations in terrain along the signal propagation path, as well as short-term multipath fading due to reflections from features such as buildings which cause fluctuations in received signal strength and other distortions at a receiving station. Mobile terrestrial radiotelephone communications are particularly susceptible to multipath fading because the signal pathways tend to be close to the ground. Doppler shift-induced (Rayleigh) fading is also problematic for receivers and transmitters moving at high speeds relative to one another.
As signals from diverse paths typically exhibit uncorrelated fading, they may be combined in the receiver to ameliorate fading effects. Similarly, radiotelephone communications signals may be transmitted and received using diverse polarizations and combined at the receiver to take advantage of the low correlation of fading between signals of differing polarizations. Despite the theoretical advantages of these reception techniques, however, sufficient diversity gain may not be possible because of limitations on antenna placement.
As illustrated in FIG. 1, a sequence of pilot symbols is time-multiplexed with information symbols, where the information symbols carry voice, data or other information. This technique is often referred to as pilot symbol assisted modulation. Symbol sequence 515 is communicated over the radiotelephone communications channel 72. A despreader 630 separates pilot symbol data 632 corresponding to the transmitted pilot symbols from information symbol data corresponding to the transmitted information symbols. The estimated transfer characteristic is then used in a Channel Estimator 640 to compensate estimation of the information symbols in response to distortion induced by the radio communications channel 72.
Pilot symbol assisted modulation can provide improved immunity to fading, but may have several drawbacks. In general, the error probability of symbol estimates tends to increase for those symbols which are most removed in time from the pilot symbols in the symbol sequence, contributing to the overall bit error rate for radiotelephone communications communicated over the channel. In order to reduce the bit error rate, pilot symbols may be inserted in the symbol sequence at smaller intervals to reduce the separation between the pilot symbols and to increase the accuracy of the estimated channel transfer characteristic. As pilot symbols generally have no information content, however, increasing the frequency of pilot symbols in the transmitted symbol sequence can reduce the potential information capacity of the channel, which may in turn reduce the number of channels a system can provide and the quality of each channel. Adding pilot symbols may also reduce power efficiency by wasting transmit power on non-informational symbols. While time-multiplexed pilot symbols allow down link adaptive antennas at the base station to be implemented, embedded pilot symbols are few in number and have only the same energy as the rest of the data symbols, so challenges persist
Addressing the shortcomings of using time-multiplexed pilot symbols has taken on added commercial import because of the planned widespread adoption of W-CDMA as a next generation wireless communication standard, wherein time-multiplexed pilot symbols are to be used.
There are a number of available approaches to improving channel transfer characteristic estimation in embedded pilot systems. Two particularly effective available approaches include Weighted Multi-Slot Averaging (WMSA), described in "Channel Estimation Using Time Multiplexed Pilot Symbols for Coherent Rake Combining for DS-CDMA Mobile Radio" by Andoy, et al., 1997, incorporated herein by reference, and Iterative Channel Estimation (ICE), described in The Use of Iterative Channel Estimation (ICE) to Improve Link Margin in Wideband CDMA Systems", by Sclimidl, et al., 1999, incorporated herein by reference.
FIG. 2 illustrates the WMSA technique and operational principals, intended to improve upon using a single pilot symbol for channel transfer characteristic estimation (instantaneous channel estimation). WCDMA downlink signals are transmitted in frames of duration 10 ms and comprise 15 time slots with duration 0.66 ms. For most data rates, there are four pilot symbols per time slot. One way to estimate the channel transfer characteristic is therefore to average the pilot symbols from multiple time slots, thereby reducing the deleterious effect of random errors and signal aberrations. Typically, 2*K sets of pilot symbols are averaged, with K sets preceding and K sets following the data symbols of the current time slot as shown in FIG. 2. The WMSA name is derived from the different weights applied to the pilot symbols depending on the distance from the time slot under analysis. If the Doppler rate were known, then the ideal coefficients could be found using a known filter. For K=2, the standard, coefficients experimentally derived to provide good performance over a wide range of Doppler frequencies are 1.0 for near symbols and 0.6 for distant symbols, as shown and well known in the art. For K=3, the optimal coefficients for near, medium and far are typically known to be 1.0, 0.8, and 0.3, respectively.
One problem not addressed by WMSA is revealed in high fading situations where there may be considerable shifts in fading from one pilot segment to another within adjacent time slots. This shortcoming of WMSA led to the development of Iterative Channel Estimation (ICE), wherein both pilot and data symbols are used to estimate channel transfer characteristics. The word "iterative" is used since an initial channel estimate is made using only the pilot symbols, and then channel estimates are refined in one or more iterations by using both pilot and data symbols.
FIG. 3 illustrates the known ICE method in block diagram format. In the first (0th) iteration, the standard WMSA channel transfer characteristic estimation method is used. The estimation derived from the 0th iteration is then used to remove the data modulation for the symbols in the shaded block. Next, as illustrated in FIG. 4, a sliding average window using both pilot and data symbols are used to make subsequent estimations. It should be noted that the channel estimate illustrated in FIG. 4 applies only to second and later iterations, and is calculated for the symbol in the center of the window. In actuality, there is a different channel estimate for each symbol. The number of symbols used in the moving average is chosen according to the approximate Doppler rate. For high Doppler rates, one time slot of symbols on either side of the chosen symbol is averaged, and for low Doppler rates two time slots of symbols are averaged. An estimate of the approximate Doppler frequency is performed before, typically using WMSA, so this represents no additional complexity for the ICE-enabled receiver.
There remain, however, significant shortcomings of both the WMSA and the ICE methods, particularly with white noise and extremely high Doppler rates. With WMSA, a random noise error effect can corrupt an entire data frame. With ICE, feedback error leads to diminished improvement over WMSA at high data rates. There is, therefore, a need in the art for an improved method of estimating channel transfer characteristics for WCDMA and other received wireless communications signals.